Process for elimination of sulfur from metals



United States Fatent O PROCESS FOR ELHVIINATION OF SULFUR FROM METALSJoseph H. Brennan, Niagara Falls, and Cecil G. Chadwick,

Lewiston, N.Y., assignors to Union Carbide Corporation, a corporation ofNew York No Drawing. Filed Sept. 26, 1958, Ser. No. 763,426

14 Claims. (Cl. 75-1305) This invention relates to an improved processfor the removal of sulfur from metals.

vOne of the important problems involved in the making of commercialalloys, such as terrochromium, is the production of suitable low-sulfuralloys. In the present state of the art, low-carbon, low-sulfurferrochromium may be produced by reacting high carbon ferrochromium withsilica according to the method disclosed by H. deW. Erasmus in US.Patent No. 2,473,019. While this method is eiiective in substantiallyeliminating the sulfur content, it produces a low-carbon, ferrochromiumalloy containing large amounts of silicon. When the high carbon'ferrochromium is decarburized with oxides other than silica, only asmall part of the sulfur content is removed.

It is, therefore, an important object of the present invention toprovide a simple, inexpensive and effective method of substantiallyeliminating the sulfur of sulfurbearing metals and alloys without theintroduction of undesirable elements.

Another object of the present invention is to provide an improved andsimplified method of removing sulfurous contaminants from metals andalloys during the vacuum-heating thereof.

Another object of the present invention is to provide a novel mixturecapable of employment with the novel method of the invention to producea low-sulfur ferrochromium alloy of improved quality, which issubstantially free from the effects of silicon.

According to the broad aspects of the invention, a sulfur-containingmetal or alloy, such as ferrochromium, is intimately mixed with tin orother metals capable of forming a volatile sulfide compound. Undervacuum heating conditions, the tin reacts with sulfur in the solid stateat temperatures well below the melting point of the alloy, to form areadily removable, volatile, stannous sulfide. Having a low boilingpoint, tin is especially suitable in the practice of the invention,since the excess or uncombined tin may be readily volatilized attemperatures well below the temperatures normally encountered in vacuumheating.

The sulfur-bearing metal or alloy to be treated ac cording to the methodof the invention may have a high or a low carbon content and may containany desired constituents, as for example, metals such as chromium ornickel. For simplicity, the invention will be described in connectionwith the decarburization of high carbon ferrochromium, using tin as theaddition agent, although it is to be understood that the invention isnot limited thereto, but is susceptible of application in othersulfurbearing metals and alloys with various suitable metal additionsother than tin.

The tin to be used in the method of the invention may be used in itselemental form, as a constituent of the chromium alloy, or as a separatealloy or ferroalloy, or even as the common ore of tin cassiterite,depending upon the specific conditions desired. For example, a ferrotinalloy may be used in conjunction with the high ICC carbon sulfur-bearingferrochromium to constitute the respective sources of tin andferrochromium material in the method of the invention.

According to the present invention, improved sulfur removal in thevacuum heating of high carbon, sulfurbearing ferrochromium may beattained by reacting the sulfur with tin in the solid state to form avolatile sulfide of tin. To this end, the high carbon ferrochromium andtin are proportioned within certain prescribed limits, so that the tinreacts with substantially all of the sulfur contaminants in theferrochromium.

For efliciently utilizing the reactivity between the tin and the sulfur,the amount of tin present in the ferrochromium should be at least equalto that required for complete stoichiometric reaction with the containedsulfur. Preferably, an excess quantity of tin may be employed, over andabove the stoichiometric amount required, the excess being readilyvolatilized at the decarburization temperatures used in the practice ofthe invention.

In a preferred practice of the invention, the high carbon ferrochromiumand the tin or a grindable alloy of tin may be finely comminuted to 325mesh (0.044 mm. openingslor smaller, and mixed with chrome ore or othersuitable oxide until a substantially homogeneous dry mixture isattained. A compact of this mix may then be suitably bonded, using, forexample, a cereal binder such as Mogul and water in an amount suflicientto moisten and plasticizethe mix. The moistened ingredients may becompacted into pellet form and dried by mildly heating at about 200 C.Thereafter, the pelletized material may be decarburized by vacuumheating at a pressure of about 2 mm. Hg. absolute and a temperaturesufiicient to provide a reaction in the solid state but below the fusionpoint of any of the charge, preferably at about between 1100 C. and 1450C.

As an alternative, the tin may be alloyed with high carbonferrochromium, either by simple addition to the pots receivingferrochromium from the smelting furnace, or as an alloying additionduring the smelting of the high carbon ferrochromium charge. If addedduring smelting, the tin additive material may be in the form ofmetallic tin, 'ferrotin, or the mineral cassiterite.

The following examples are illustrative of the practice of theinvention, and more fully indicates the extent of the desulfurizationeffect of tin on high carbon ferrochromium during vacuum heatingdecarburization treatment.

EXAMPLE I Lbs. Ferrochromium 1800 Tiebaghi chrome ore 700 Mogul 40 Thechrome ore had the following approximate analysis:

Percent Or O 54 FeO 14 A1 0 10 Si0 3 MgO 16 CaO 1 Sufficient water wasadded to Slightly moisten the mixture Table 1 LC. Ferrochromium ProductProduced in Accordance with the Invention H.C. Ferroehromium V ChargePercent Percent Sn c Fe and Incidental Impurities balance EXAMPLE II Ina series of tests similar to the test conducted in Example I,varyihgadditions of tin were made in the form of cassiteriteconcentrates containing 71.88 percent tin. In these tests the tin wasless than the stoichiometric amount (0.23 percent by weight) required toreact with balance horizontal vacuum furnace at a temperature below thefusion point of any of the charge. The furnaced pellets all the sulfurin the ferroc'hromium. Before treatment the highcarbon ferr'ochromiuincontained 0.064 percent sulfur. The analyses of the products obtainedare indicated in Table II. v

Table II Preduclt Constituent,

Tin Addttlonf Percent wt. of 11. 0. FeCr ercent Top The above testresults indicate that the combination of tin and high carbonferrochromium is admirably suited in substantially lowering the sulfurcontent during the decarburization of high carbon ferrochromium, withoutany significant carryover of the tin addition in the resultant lowcarbon ferrochromium product. Because of the reactivity of tin andsulfur in the solid state under 'vacuum heating conditions, and the lowboiling tempera- To illustrate the procedure wherein tin compounds arewere then analyzed for sulfur, carbon, and tin. The results are shown inTable III.

Table 111 Product Constituent, percent Desulturlzlng Tln Compound TopBottom.

0- 0.007 0.005 Stannous chloride, 51101221120 S I 0.020 0.028 Sn 0.0010. 001 C 0.009 0.005 Stannic Oxide, S 8 0.025 0.013 sn 0.001 0.001 C0.012 0.012 Stannous chromate, Sn(CrO4) S 0.012 0.015

s 0. 001 0,001 C 0.008 0.003 'Iln concentrates, 70% Sn as SnOLn, S 0.035 0.017 Sn 0.001 0.001

From these data it may be seen that tin may be added inthe form of tincompounds as well as, in the form of alloys and elemental metal for theremoval of sulfur from the low ferrochrome product provided that thecompound, ore, alloy or elemental metal does not contain sulfur, i I

It 'will be understood that modifications and. variations may beeffected without departing from the spirit and scope of the novelconcepts of the present invention.

This application is a continuation-in-part of United States patentapplication Serial No. 605,925, filed August 2.4, 1956, now abandoned.

What is claimed is:

1. In the art of manufacturing a low-carbon sulfurfree ferrochromiumalloy from a high-carbon sulfurbearing ferrochromium alloy, theimprovement which comprises comminuting said high-carbon ferrochromiumto a particle size of less than 325 mesh, mixing therewith at least onesimilarly sized sulfur-free material selected from the group consistingof elemental tin, tin alloys, tin compounds, and tin-containing ores, inan amount suflicient to react with all of the sulfur in said highcarbonferroch-romium, pelletizing the admixture of saidhigh-carbon'ferrochromium and said selected material, and decarburizingthe said pelletized material under vacuum heating conditions for a timesufficient to react in "the solid state the tin of said selectedmaterial with sub stantially all of the sulfur in said high-carbonferrochromium to form a volatile sulfur compound and to vaporize anyexcess contained tin.

2. In the method of desulphurizing a sulfur-c0ntain ing ferrochromiumwherein said ferrochromium is comminuted and mixed with a comminutedoxidant, and the mixture so formed is heated under subatmosphericpressure, the improvement comprising admixing, prior .to heating undersubatmospheric pressure, at least one oomminuted sulfur-free materialselected from the group consisting of elemental tin, tin alloys, tincompounds, and tin-containing ores, in an amount approximately requiredto react with all of the sulfur in the solid state during said heatingto form a volatile sulfurstin compound.

3. In the solid state, vacuum' heating of sulfur-Icontaining materialsselected from the group consisting of metals and alloys toeffect'desulphurization. thereof, the improvement which comprisesproviding, in a corn- .minuted mixture containing said selectedgroupmember, an amount of atleast one sulfur-free material selected fromthe group'co nsisting of elemental tin, tin alloys,'

tin compounds, and tinrcontaining ores capable ofreacting, duringsaid-heating, with. substantially all ofsaid contained sulfur to form avolatile sulfur-tin compound without causing a substantial amount of tinto remain in the resultant product. v

4. In the manufacture of a sulfur-free metal from; a

sulfur-containing metal, the improvement which comprises incorporatingwith said metal at least one sulfurfree material selected from the groupconsisting of elemental tin, tin alloys, tin compounds, andtin-containing ores, containing an amount of tin approximatelysufficient to react with all of the contained sulfur, and heating saidmetal and selected group material at a pressure and a temperature, belowthe fusion points of said metal and tin, suflicient to react saidcontained tin and said sulfur in the solid state to form a volatile tinsulphide compound and to vaporize any excess contained tin.

5. An improved process in accordance with claim 2 wherein saidcomminuted sulfur-free material is ferrotin.

6. An improved process in accordance with claim 2 wherein saidcomminuted sulfur-free material is tin oxide.

7. An improved process in accordance with claim 2 wherein saidcomminuted sulfur-free material is elemental tin.

8. An improved process in accordance with claim 2 wherein saidcomminuted sulfur-free material is stannous chloride.

9. An improved process in accordance with claim 2 wherein saidcomminuted sulfur-free material is stannous chromate.

10. An improved process in accordance with claim 4 wherein saidcomminuted sulfur-free material is ferrotin.

11. An improved process in accordance with claim 4 wherein saidcomminuted sulfur-free material is tin oxide.

12. An improved process in accordance with claim 4 wherein saidcomminuted sulfur-free material is elemental tin.

13. An improved process in accordance with claim 4 wherein saidcomminuted sulfur-free material is stannous chloride.

14. An improved process in accordance with claim 4 wherein saidcomminuted sulfur-free material is stannous chromate.

References Cited in the file of this patent UNITED STATES PATENTS2,238,194 Tainton Apr. 15, 1941 2,473,019 Erasmus June 14, 19492,626,863 Erasmus Jan. 27, 1953 FOREIGN PATENTS 729,177 Great BritainMay 4, 1955

1. IN THE ART OF MANUFACTURING A LOW-CARBON SULFURFREE FERROCHROMIUMALLOY FROM A HIGH-CARBON SULFURBEARING FERROCHROMIUM ALLOY, THEIMPROVEMENT WHICH COMPRISES COMMINUTING SAID HIGH-CARBON FERROCHROMIUMTO A PARTICLE SIZE OF LESS THAN 325 MESH, MIXING THEREWITH AT LEAST ONESIMILARLY SIZED SULFUR-FREE MATERIAL SELECTED FROM THE GROUP CONSISTINGOF ELEMENTAL TIN, TIN ALLOYS, TIN COMPOUNDS, AND TIN-CONTAINING ORES, INAN AMOUNT SUFFICIENT TO REACT WITH ALL OF THE SULFUR IN SAID HIGHCARBONFERROCHROMIUM, PELLETIZING THE ADMIXTURE OF SAID HIGH-CARBONFERROCHROMIUM AND SAID SELECTED MATERIAL, AND DECARBURIZING THE SAIDPELLETIZED MATERIAL UNDER VACUUM HEATING CONDITIONS FOR A TIMESUFFICIENT TO REACT IN THE SOLID STATE THE TIN OF SAID SELECTED MATERIALWITH SUBSTANTIALLY ALL OF THE SULFUR IN SAID HIGH-CARBON FERROCHROMIUMTO FORM A VOLATILE SULFUR COMPOUND AND TO VAPORIZE ANY EXCESS CONTAINEDTIN.